During the manufacture of printed circuit boards there exists a need for a means to accurately hold a printed circuit board in position during various steps in the manufacturing process including a step known as solder stenciling. Solder stenciling requires that the printed circuit board be placed within a solder stenciling machine where a portion of the solder stenciling machine is lowered onto the printed circuit board from above in order to deposit solder onto the copper leads of the printed circuit board. In order to hold the printed circuit board in position during the solder stenciling process, pallets are made by machining a suitable material to create a recessed space in the pallet for receiving the printed circuit board. The pallets will vary in size and thickness depending on the size of the printed circuit board which the pallet is to hold. When positioned within the recessed space the printed circuit board lies flat and level with or beneath the plane of the top surface of the pallet. The printed circuit board must be held in position so that it does not move from side to side or move upward and out of the recessed space. If the board were to bend, the position of the copper leads to receive solder would be moved and the process would not be properly accomplished. Therefore, a way to apply pressure to the printed circuit board to hold it in place without deformation is needed. Clamps which protrude above the surface of the top of the pallet will not work for solder stenciling because the solder stenciling process requires that the stencil machine come down directly over the printed circuit board and then lift up after depositing the solder paste. The solder and the machine will exert a slight suction or adhesion which can lift the board up out of the recessed cavity if the pallet does not have a means to hold the board in position.
Since pallets must be designed for each specific size and type of printed circuit board to be manufactured and since the pallets are reused numerous times, an apparatus for positioning and holding the printed circuit board that is simple, reliable, easy to manufacture and inexpensive is needed.
One example of the present methods of holding the printed circuit board in position during the solder stenciling process are pallets made by Precision International, Ltd. Precision International, Ltd. makes pallets in which a cavity is cut for holding a spring. A lid is placed over the spring and screwed shut so that the spring is contained inside the pallet and the top surface of the pallet is flat. The spring then applies pressure to the printed circuit board. This device, while effective, requires extra steps in the manufacturing process and extra handling for assembly. A need exists for an apparatus for holding the printed circuit board in place without the additional parts and the additional manufacturing steps entailed in the Precision International, Ltd. pallet.
U.S. Pat. No. 3,738,639 discloses a coiled spring attached to a base which holds the substrate against a fixed stop. The coiled spring does not meet the need for a pallet without additional parts and assembly steps.
U.S. Pat. No. 4,201,374 discloses a substrate carrier with spring arms for positioning the substrate with the hooked ends of the spring arms. The spring arms are engaged to an external actuating means for retracting and inserting the arms. Like the coiled spring discussed above, the apparatus, if applied to a reflow pallet, does not meet the need for a reflow pallet without additional parts and assembly steps.
U.S. Pat. No. 4,425,076 discloses a process and apparatus for automatic positioning of a substrate on a worktable. The patent discloses two pivoting arms driven by a motor for positioning a work piece. The apparatus is too complicated for economical application in the solder stenciling process.
U.S. Pat. No. 4,582,309 discloses jigs for locating and holding circuit boards in a desired location by means of four accurately formed holes on the printed circuit board which align with pins in the jig, one of which pins is chamfered to align with one corner of the board. The pins are directed to a different assembly step than reflow soldering and while the pins provide stability in the horizontal plane they do not provide stability in the vertical plane. Moreover, the pins protrude above the printed circuit board and are therefore not suitable for reflow soldering.
U.S. Pat. No. 4,593,804 discloses an apparatus for guiding a circuit board onto a test fixture by means of a circuit board holder with guide pins that protrude upward above the printed circuit board. The pins do not provide stability in the vertical plane and since they protrude above the printed circuit board are not suitable for a pallet to hold a printed circuit board during the reflow soldering process.
U.S. Pat. No. 4,684,113 discloses a universal holding fixture where the workpiece is supported by rods through which a holding force is generated by a vacuum. While the holding force solves the problem of stabilizing the printed circuit board in the vertical plane, the addition of a machine to generate a vacuum makes this apparatus unsuitable for a reflow pallet that is easy to manufacture and inexpensive.
None of the above disclosures meet the need for a device that can simultaneously hold a printed circuit board in both the horizontal and vertical planes without without parts that protrude above the surface of the printed circuit board and without additional costly assembly steps in the manufacture of the reflow pallet.
Therefore, a need exists for a reflow pallet that is inexpensive to manufacture, easy to use, reliable, accurate, reusable, capable of properly aligning the printed circuit board and capable of stabilizing the printed circuit board in both the horizontal and vertical planes.